The present invention relates to electric meters and, more particularly, to demand registers for electric meters.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed which is prportional to the electric power being consumed by a load. Geared dials, or cyclometer discs, integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
In addition to the above measurement of consumption, some electric meters contain demand registers having means for separating the consumption into those parts of consumption occurring during peak and off-peak hours (however defined) and for recording maximum demand during any one of a contiguous set of demand intervals over a predetermined period of time in order to adjust billing according to such parameters. In one such meter disclosed in U.S. Pat. No. 3,586,974, a mechanical demand register records the maximum demand during a predetermined period of time and stores the value for reading. The predetermined period of time may be, for example, the time between meter readings, or a period of time corresponding to the billing period of the utility providing the power. A clockwork mechanism restarts the demand register at the end of each demand interval which may be, for example, a fraction of an hour, so that, at the end of the predetermined period, the stored value represents the highest value of power usage occurring during any one of the demand intervals in the predetermined period.
Demand registers of the mechanical type, such as disclosed in the above U.S. Patent, have limited flexibility. Once their design is completed for a particular meter physical configuration, the design is not transferrable to a meter having a different physical configuration. Also, the demand-measurement functions cannot be redefined without major mechanical redesign.
When a power outage occurs, the clockwork motor driving the demand register is halted. Following the restoration of power, the clockwork motor driving the demand register is restarted without recognizing the fact that a power outage has occurred. There is some question whether resumption of operation can be performed in this way while strictly abiding by the regulations for demand metering as defined by the commissions having jurisdiction over the utility providing the electric power and without penalizing a customer for an apparent very high demand which is the result of the power outage and resumption and not the fault of the customer.
Maximum demand metering is conventionally defined as the maximum amount of power consumed in any one contiguous demand interval during the time interval of interest; that is, the maximum amount of power consumed in any one of the intervals of, for example, 15 minutes, 30 minutes or one hour. If the accumulation of actual usage during one of such demand intervals is paused by a power outage and then resumed, the demand accumulated at the end of the interrupted time period consists of an initial portion before the outage interruption and a final portion following the outage interruption. Not only does the intervention of the outage interruption appear to violate the definition of demand metering, but also, if the customer load includes a substantial number of motors or other devices having a large starting load compared to their running load, when power is restored, all or most of such motors and devices may be forced to go through their starting sequence at the same time. Under normal conditions, some of the customer's motors may run continuously and others may cycle on and off in an unsynchronized manner. Thus, the motor starting loads are normally distributed over time and their effect on demand metering is similarly distributed over time. As is well known, the starting load imposed by a motor is much higher than the running load. By synchronizing all or most of the customer's starting load in a single time period following restoration of power, an unnaturally high demand may be registered in that time period when, in fact, the unnaturally high demand is the result of the utility permitting a power outage to occur and not the result of the customer load producing the registered maximum demand.
In addition to the above limitations of mechanical demand metering, a useful demand metering technique known as rolling demand is not practically feasible using mechanical demand registers. In rolling demand metering, a demand interval is divided into N contiguous subintervals. The usage during each demand subinterval is summed with the demand recorded during the preceding N-1 subintervals. At the end of each subinterval, the total demand recorded is the demand for N subintervals, i.e. for the entire preceding demand interval. The maximum demand may then be taken as the maximum over any interval sensed at the end of a subinterval. The use of such rolling demand metering avoids distortion in the billing data which could otherwise occur due to short-term extremes in the usage data which would otherwise become lost in the averaging process over an entire demand interval.
Greater flexibility in demand metering may be obtainable using electronic acquisition, integration and processing of power usage. An electronic processor such as, for example, a microprocessor, may be employed to manage the acquisition, storage, processing and display of the usage and demand data. U.S. Pat. Nos. 4,179,654; 4,197,582; 4,229,795; 4,283,772; 4,301,508; 4,361,872 and 4,368,519, among others, illustrate the flexibility that electronic processing brings to the power and energy usage measurement. Each of these electronic measurement devices includes means for producing an electronic signal having a characteristic such as, for example, a frequency or a pulse repetition rate, which is related to the rate of power usage. The electronic processor is substituted for the mechanical demand register of the prior art to keep track of the demand during defined periods of time.
An electronic processor of an electronic demand register conventionally employs volatile random access memory for the high speed and low power consumption characteristics offered by such devices. When a power outage occurs, if steps are not taken to prevent it, all data in such volatile storage may be lost. This could include both prior demand data as well as a sense of where, in a time period, the outage occurred. The loss of demand data has an obvious negative impact on billing. The loss of time sense would require that all users in the area affected by the power outage must begin a new time period for accumulation of demand. Such synchronization of demand metering is considered undesirable by at least some utilities. U.S. Patent application Ser. No. 599,736 filed on the same date as the present application, the disclosure of which is herein incorporated by reference, discloses means for storing data in non-volatile memory when a power outage occurs and for ignoring or tolerating certain normal deviations of the line power, such as, for example, momentary overvoltage, surges, noise and momentary power outages enduring for a very short time period. This new capability to store demand billing data and programmed constants in non-volatile memory permits re-thinking the manner in which demand data accumulated before and after an outage interruption may be handled so as to provide fairness both to the consumer and to the utility.